The present invention relates to generation of pseudo random sequences and, more particularly, to generation of pseudo random sequences in a reverse link modulator.
Many electronic systems, such as wireless communication systems, use the Quaternary Phase Shift Keying (QPSK) technique to modulate and demodulate incoming binary input data stream.
FIG. 1 illustrates the four phases I, Q, {overscore (I)} and {overscore (Q)}, forming the I-Q plane of a data channel. The QPSK technique uses these four phases to convert the random binary incoming data into a noise-like waveform by modulating the incoming data using a pseudo random sequence of bits. The modulation is carried out at the clock period of the pseudo random sequence generator.
As shown in FIG. 1, the four phases I, Q, {overscore (I)} and {overscore (Q)} have respective phase differences of 0xc2x0, 90xc2x0, 180xc2x0, and 270xc2x0 with respect to the I-axis. Data coming through the I and Q channels of a reverse link modulator are identified by their positions along the I-Q plane. Furthermore, as shown in FIG. 1, such data may only assume positions at points A, B, C and D, which are respectively located along 45xc2x0, 135xc2x0, 225xc2x0 and 315xc2x0 of the I-axis of the I-Q plane.
Points A, B, C and D, have respective coordinates of (1, 1), (1, xe2x88x921), (xe2x88x921, 1) and (xe2x88x921, xe2x88x921), which are the only coordinates in the I-Q plane in which incoming data may reside.
Currently known systems do not impose restrictions on data transitions along the I-Q plane. Hence, a transition from, for example, point A to point C, in which both the I and Q coordinates of the data change, is allowed. Such a transition requires passing through the center of the I-Q plane and thus results in modulation of the carrier signal amplitude through a zero value, also referred to as zero crossing.
The greater the number of zero crossings, the higher is the degree of modulation of the carrier signal amplitude and the larger is the variation in the peak-to-average signal ratio. The large variation in the peak-to-average ratio, results in the relatively smaller battery life of the existing wireless devices (e.g. cellular telephones).
Therefore, a need continues to exist for a QPSK technique that has a reduced average-to-peak ratio and accordingly provides improved battery life.
A reverse link modulator, in accordance with one embodiment of the present invention, includes, among other components, a random sequence generator for generating a sequence of random pair of bits to modulate the incoming data bits. With each period of the pseudo random sequence generator clock, one bit of the pair of random bits changes.
The reverse link modulator also includes a long code mask, a long code generator, a complex multiplier and a pair of pulse shaping low-pass filters.
In some embodiments, the random sequence generator is a 2-bit state machine having four states (00), (01), (10), (11). The output signals generated by the state machine are the same as its states. Only transitions changing one bit of the 2-bit states are allowed.
The reverse link modulator, in accordance with the present invention, inhibits zero transitions of the spreading sequence. Therefore, the reverse link modulator has a suppressed carrier signal amplitude modulation and a reduced peak-to-average signal ratio.